Junction Potential - Online Tutor, Practice Problems & Exam Prep

A junction potential forms at the interface of two ionic solutions, influenced by ion concentration and mobility. In electrochemical cells, a salt bridge balances electron flow by allowing anions to migrate from the cathode to the anode, creating a negligible voltage. The mobility of ions, such as H⁺ and Br^-, varies due to size, affecting potential buildup. Using ions of similar size, like KCl, minimizes junction potential, ensuring accurate cell potential calculations, represented by the equation E_cell = E_cathode - E_anode.

Junction Potential

So here, we state that a junction potential is created at the interface between two ionic solutions. Now, realize from our previous discussions on electrochemical cells that we have the salt bridge. And the salt bridge's purpose is to help create a counterbalance to the electrons that travel from the anode to the cathode by releasing anions that flow from the cathode side to the anode side. This opposite movement of negatively charged ions helps to complete the circuit for our given electrochemical cell in terms of a galvanic cell or voltaic cell. Now, we're going to say at both ends of the salt bridge, we have the building up of these ions.

And depending on how quickly these ions pass as they move from the salt bridge to the solution, there is a certain amount of potential that builds up and this is when we talk about the junction potential. Now, we're going to say here that this junction potential is based on two things. It's based on the concentration of the solutions and differences in the mobility of the ions. Now, here we're going to say this creates a negligible amount of voltage at the end of the salt bridge connecting the two half reactions. So let's take a look at our image on the left.

In this image, we have two solutions that are separated from one another by a semi-permeable membrane, which means we have the movement of ions from one side to the other side. On the left side, we have a concentration of 1 molar. On the right side, we have a concentration of 0.1 molar. Now what's going to happen is our ions will travel to the side that is less concentrated. So this is a form of dispersion where we go from high concentration to low concentration.

So what's going to happen is our H⁺ ion and our Br^- ions will traverse the semi-permeable membrane and go from the area of high concentration to the area of low concentration. Notice the difference in the lengths of the arrows. That's because these two ions are not the same size. So, they're going to both travel at different rates towards the right side. Because hydrogen is smaller, it'll move faster.

What does this cause in terms of my semi-permeable membrane? Well, realize here that we have positive ions that are crossing over much more quickly than negative ions. So there's going to be a buildup of positive ions here and that's because hydrogen has crossed over faster. So, more of them are going to cross there which helps to build up the amount of H⁺ on that side. So, it will be positively charged in terms of the right side of my semi-permeable membrane.

At the same time, my bromide ions are bigger and slower. So, they are going to be left behind. There's going to be a slow buildup of bromide ions because again the H⁺ ions are leaving quickly enough and they are not leaving quickly enough that there's going to be a buildup of negative ions on the left side of my semi-permeable membrane. This here is an example of a buildup of potential in terms of the semi-permeable membrane. This is what we expect when we are dealing with our salt bridge and the possibility of a junction potential being built up.

Now, here we're accustomed to seeing that the cell potential, which is E_cell = cathode minus anode or in this case, indicator cell versus the reference electrode. So, the indicator electrode versus the reference electrode. We also have to take into account the junction potential that can result as my ions move from one side to the other side. Normally, we don't include this junction potential here because we use the right types of ions within my salt bridge to minimize the degree of this buildup of potential. Now, when we're talking about mobility of ions, we're talking about that based on the size of the ions themselves.

The bigger the ion is, the slower it will move. So, as we can see here, we have the mobility of different ions. In our example, we have the hydrogen ion which has a mobility of 36.30 × 10^-8, and we have bromide ions, which have only 8.13 × 10^-8. H⁺ is smaller than Br^- which is why it moves faster. We're also going to see that we have junction potentials that can result based on the concentrations as well as the identity of the ions used.

Now, we're going to say here, we can see that there's a building up of potentials as we compare different ions to one another and different concentrations to one another. What this is showing me is that when it comes to creating the perfect junction potential, it's best to use KCl because the two ions have similar mobility values. So, there's going to be a small buildup of potential. So small that it's negligible and the sub-potential overall is just again cathode minus anode or in this case, the indicator electrode versus the reference electrode. Here, you want to make sure you choose ions that are similar in size so that you have a potential that is as small as possible.

From the options given here, we can see that the smallest difference in potential is given in this option here. And again, when it comes to the right types of ions used for my salt bridge, potassium chloride is the best type of ionic compound to use because K⁺ and Cl^- are so close in size. Now that we've talked about this, attempt to do the example problem left on the bottom of the page. Based on the junction provided, determine which side will become more negative.

Junction Potential

So here it asks which side of the 0.5 molar sodium bromide or 0.5 molar potassium bromide junction will be more negative. Alright, so we have our junction here or semi-permeable membrane here. We have 0.5 molar sodium bromide on this side, and we have 0.5 molar potassium bromide on this side. Realize here that both of these contain the same negative ion, the bromide ion. So that means it's not going to play a factor because both sides have bromide ions and both have the same concentration of bromide ions.

So, really the difference is in looking at the positive ions. There are different ions, so they're going to move at different rates, and that's the key to determining which side will be more negative. If we take a look here, we have the potassium ion with the mobility of this, and we have the sodium ion with the mobility of this. We can see that the potassium ion is actually moving faster here. Now because it moves faster, it's going to be able to cross the semi-permeable membrane at a faster rate, and sodium is moving much slower.

So it's not going to be able to cross over to the other side, this side here of the semi-permeable membrane as quickly. So what's going to happen is we have some sodium ions that are left back here because they can't move fast enough, and then we also have potassium ions crossing over much more quickly. So we're going to have a buildup of positive ions on this side. So overall, the left side would have a higher concentration of positive ions. So that side would become more positive, and if the left side is more positive that would mean that it's this side here, the side with the potassium bromide, that will be more negative.

So when it comes to questions like this, we have to look at two things. We look at concentration and we look at the ions themselves. The one with the greater concentration is going to have ions leaving that side to go to the other side at a faster rate. But if the concentrations are tied, we then look at the number of ions, the type of ions. The higher the mobility of that ion, the faster it can cross over the semi-permeable membrane and create a charge imbalance on the other side of the semi-permeable membrane or junction.

Here we can see that potassium and sodium definitely move at different rates. Because potassium can move faster, it's going to cross over to the left side and make that side more positively charged. Here we're looking for which side is more negative. So if the side with the sodium ion is more positive, by default, the side with the potassium would have to be more negative. More ions are leaving that side at a quicker rate, leaving it depleted of positive ions and therefore more negative.

Here’s what students ask on this topic:

What is junction potential in electrochemical cells?

Junction potential arises at the interface between two ionic solutions in electrochemical cells. It is influenced by the concentration and mobility of ions. In a galvanic or voltaic cell, a salt bridge helps balance electron flow by allowing anions to migrate from the cathode to the anode. This movement creates a negligible voltage, ensuring the circuit is complete. The potential difference is due to the varying speeds at which different ions, such as H⁺ and Br^-, move across the interface. This potential is minimized by using ions of similar size, like KCl, to ensure accurate cell potential calculations.

How does ion mobility affect junction potential?

Ion mobility significantly affects junction potential because different ions move at different rates due to their sizes. Smaller ions, like H⁺, move faster than larger ions, like Br^-. This difference in mobility causes an imbalance in charge distribution at the interface, leading to a potential difference. For example, in a semi-permeable membrane, H⁺ ions will cross faster than Br^- ions, causing a buildup of positive charge on one side and negative charge on the other. To minimize junction potential, ions with similar mobilities, such as K⁺ and Cl^-, are used in salt bridges.

Why is KCl commonly used in salt bridges to minimize junction potential?

KCl is commonly used in salt bridges because the mobilities of K⁺ and Cl^- ions are very similar. This similarity ensures that both ions move at nearly the same rate, minimizing the buildup of potential at the junction. The negligible junction potential allows for more accurate measurements of the cell potential, represented by the equation E_cell = E_cathode - E_anode. Using KCl helps maintain the integrity of the electrochemical cell's readings by reducing the impact of ion mobility differences.

How is junction potential related to the concentration of ionic solutions?

Junction potential is directly related to the concentration of ionic solutions. When two solutions of different concentrations are separated by a semi-permeable membrane, ions will move from the higher concentration to the lower concentration. This movement creates a potential difference due to the varying speeds at which different ions migrate. For instance, in a solution with 1 M H⁺ and 0.1 M Br^-, H⁺ ions will move faster, causing a buildup of positive charge on one side and negative charge on the other. This concentration gradient contributes to the junction potential.

What role does a salt bridge play in an electrochemical cell?

A salt bridge plays a crucial role in an electrochemical cell by maintaining electrical neutrality and completing the circuit. It allows the flow of anions from the cathode to the anode, counterbalancing the electron flow from the anode to the cathode. This movement of ions helps prevent the buildup of charge that would otherwise stop the cell from functioning. The salt bridge also minimizes junction potential by using ions with similar mobilities, such as K⁺ and Cl^-, ensuring accurate cell potential measurements.